Before our fall, the demons say that God is a friend of man; but after the fall, that He is inexorable.
- Ladder of Divine Ascent, 5.31
When we are thinking about committing a sin, the demons tell us, "Oh, don't worry, God is a loving, merciful God, you can go ahead and commit the sin, He'll forgive you!" But no sooner is the sin committed, than they change their tune and tell us, "Oh, now you're in trouble! God saw that, and He is a just God, who punishes every sin! There's no way He'll forgive you for that."
Let us remember this trick of the demons. God is indeed merciful, loving and forgiving; He is also absolutely just, and will require an account from us at the judgment even of every idle word, not to mention our sinful words and deeds. But how much better for our spiritual lives if we remember the opposite of what the demons tell us, namely, when we are tempted to sin, let us remember that God sees everything, even the secret thoughts of the heart, and He will require us to give account for it; but if we fall into sin, then let us remember that God is merciful, and so be moved to repent for our sin. As one father said, the question is not, "Why did you fall?" but rather, "Why did you not get up again?"